In systems for the receiving and/or transmitting electromagnetic signals, such as, for example, radio, TV or radar systems, conversion of the electromagnetic signal's frequency is a common occurrence, for which so called mixers are used.
In such systems, mixers are used in order to shift from a radio frequency, RF, to a lower frequency, usually a so called intermediate frequency, IF, signal. Naturally, a mixer can also be used in order to shift in the other direction, i.e. from an IF signal to an RF signal, so called “up-conversion”, as opposed to “down-conversion”.
In the case where a mixer is used for shifting from an RF signal to an IF signal, the mixer is provided with two input signals, the RF signal and a local oscillator, LO, signal, and the mixer produces the IF signal as its output signal. The frequency of the IF signal, i.e. the IF frequency, fIF, is determined by the RF frequency, fRF, and the LO frequency, fLO, as described by the equation fIF=fRF−fLO. As those skilled in the field will know, a “sum frequency” fRF+fLO is also obtained in the mixer, but this frequency is usually not used.
A subharmonic mixer is a special kind of mixer, which operates using the equation fIF=fRF−2fLO. Since the LO frequency here is multiplied by two, a lower LO frequency can be used than in mixers which employ the LO signal “multiplied by one”, as in the equation fIF=fRF−fLO. The use of a lower LO frequency is often beneficial in that it gives the subharmonic mixer lower phase noise and higher output power than with the use of a higher LO frequency, which is used in mixers which use the equation fIF=fRF−fLO.
Key parameters in a sub-harmonic mixer are the conversion gain and linearity.
A subharmonic mixer's linearity is often measured by a so called two-tone test in which two RF signals with a small frequency separation between them are input to the subharmonic mixer and the intermodulation components at the output of the subharmonic mixer are measured. The amplitudes' intensities of the second-order and third-order intermodulation components are indicative of the mixer's linearity.
Known kinds of subharmonic mixers include Gilbert-type subharmonic mixers and passive subharmonic mixers. Gilbert-type subharmonic mixers exhibit poor linearity, while passive subharmonic mixers exhibit poor conversion gain.